A significant problem in fusion fuel processing is the recovery of tritium from fusion fuel (D-T) impurities such as water and methane. This is difficult because of the relative stability of these starting materials. Various methods have been used or proposed for this purpose, but most suffer from problems such as waste generation, unreliability and complexity. For a discussion of these methods, see, for example, R. S. Willms and S. Konishi, "Fuel Cleanup Systems for Fusion Fuel Processing," Fusion Engr. & Design 18, 53 (1991). Another area where recovery of purified hydrogen is required is for fuel cells, since the fuel must be free of CO to avoid poisoning the cell.
Combined reactor/permeators have been examined for various applications since the late 1960s. Typically, they consist of a plug-flow catalytic reactor with walls composed of a membrane material. The membrane walls facilitate the addition of reactants or the removal of products along the length of the reactor. This is particularly useful for reversible reactions which are limited by thermodynamic equilibrium. Products can be removed as the reaction proceeds and, with a proper membrane, reactions can be made to proceed to essentially 100% conversion.
Using shift catalysts, hydrogen can be recovered from water and methane as free hydrogen. Examples of these reactions are: EQU CO+H.sub.2 O CO.sub.2 +H.sub.2 (Water Gas Shift); and EQU CH.sub.4 +H.sub.2 O CO +3H.sub.2 (Steam Reforming).
Both reactions are reversible and are limited by thermodynamic equilibrium. By including a membrane in the reactor, which selectively removes H.sub.2 from the reacting system, the reactions can be brought to near completion. Methane and water are the principal hydrogen isotope (protium, deuterium, and tritium)-containing products from fusion reactions. Hydrocarbons, which may also be utilized in the steam reforming reaction, and water are the principal hydrogen-containing sources of hydrogen for fuel cells.
Shigeyuki Uemiya et al., in "The Water Gas Shift Reaction Assisted by a Palladium Membrane Reactor," Ind. Eng. Chem. Res. 30,585(1991), have shown that a membrane reactor provides higher levels of carbon monoxide conversion beyond the equilibrium attainable in a closed system. This is a result of a shift in thermodynamic equilibrium of the water gas shift reaction toward the product side by the selective and rapid removal of hydrogen from the reacting system. Carbon monoxide and steam at atmospheric pressure were reacted over an iron-chromium oxide catalyst heated to 673K in a reactor having a palladium membrane inner tube supported on a porous glass cylinder. The authors also discuss copper-based catalysts which must be operated at lower temperatures.
Similar improvement in the steam reforming of methane has been reported by Shigeyuki Uemiya et al., in "Steam Reforming of Methane in a Hydrogen-Permeable Membrane Reactor," Appl. Catalysis 67, 223 (1991). There, methane was reacted with steam over a Ni catalyst heated to between 623 and 773K in a reactor incorporating a palladium film supported on a porous glass cylinder.
However, there is no suggestion in these references that both reactions could be made to proceed efficiently using a single shift catalyst at a single temperature in a permeable membrane reactor.
Accordingly, it is an object of the present invention to simultaneously recover substantially pure hydrogen from water and hydrocarbons in an efficient single-step process.
Another object of the present invention is to recover deuterium and tritium from fusion reactor exhaust which contains deuterium- and tritium-bearing water and hydrocarbons.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.